Known photoconductive materials for electrophotographic light-sensitive compositions include various organic compounds some of which are known to have considerably high sensitivity. However, it is very rare at present to practically use organic photoconductive materials for electrophotographic materials.
Organic photoconductive materials or photoconductors have various excellent properties as compared with inorganic photoconductive materials or photoconductors and hence provide a wide range of technical applications in the technical field of electrophotography. For example, the production of transparent electrophotographic light-sensitive films, flexible electrophotographic light-sensitive films, light-weight and easily handling electrophotographic light-sensitive films, etc., have become possible for the first time using organic photoconductors. Also, organic photoconductors have such properties as a film-forming property during the production of electrophotographic light-sensitive materials, a surface smoothness, and a selectivity of a charging polarity for electrophotographic processes. Inorganic photoconductors do not have such properties.
In spite of having various excellent properties, organic photoconductors have not sufficiently contributed to the technical field of electrophotography up to now mainly because of their low light-sensitivity and the brittleness of the films or layers of the photoconductors.
The study of organic photoconductors was first made on compounds such as low-molecular heterocyclic compounds, nitrogen-containing aromatic compounds, and various high molecular aromatic compounds. As the results of such studies, some compounds having a considerably high sensitivity were found. However, recently sensitization methods for obtaining higher sensitivity is likely to become the center of study. This is because even the organic photo semiconductive compound known to having the highest sensitivity does not have a sensitivity such that the compound may be used, as it is, without need of the application of a sensitization treatment. Therefore, when practically using an organic photoconductor, it is necessary to select the most effective sensitizing method and apply the sensitization to the organic photoconductor. Therefore, it is clear that the industrial value of an organic photoconductor depends upon the extent of the sensitivity of the electrophotographic light-sensitive material finally obtained by the sensitizing means applied.
The most generally known sensitizing method is the addition of a sensitizing dye and the addition of a Lewis acid, which can be applied to almost all organic photoconductors. The former method sensitizes the organic photoconductor by imparting the spectral absorption characteristics of a dye to the organic photoconductor and the latter method sensitizes the organic photoconductor by forming a complex of donors and acceptors in the organic photoconductor to provide a new spectral sensitivity.